Timer-based method and apparatus for requesting SIB

ABSTRACT

Provided are a timer-based method for a terminal for requesting a system information block (SIB), and an apparatus for supporting the method in a wireless communication system. The method may comprise the steps of: determining whether an essential SIB has been omitted from a cell; requesting the omitted essential SIB from an RAN if the essential SIB is determined to have been omitted from the cell; starting a system information request timer; and re-requesting the omitted essential SIB from the RAN when the system information request timer expires.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2017/004793, filed on May 10, 2017, which claims the benefit of U.S. Provisional Applications No. 62/334,418 filed on May 10, 2016, No. 62/334,427 filed on May 10, 2016, No. 62/334,443 filed on May 10, 2016, and No. 62/366,643 filed on Jul. 26, 2016, the contents of which are all hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wireless communication system and, more particularly, to a method for a user equipment (UE) to request a missing essential system information block (SIB) based on a timer in a wireless communication system, and an apparatus for supporting the same.

Related Art

In order to meet the demand for wireless data traffic sorting since the 4th generation (4G) communication system came to the market, there are ongoing efforts to develop enhanced 5th generation (5G) communication systems or pre-5G communication systems. For the reasons, the 5G communication system or pre-5G communication system is called the beyond 4G network communication system or post long-term evolution (LTE) system.

In order to accomplish a higher data transfer rate, the 5G communication system considers implementation at a super-high frequency (mmWave) band (e.g., such as a 60 GHz band). In order to obviate a path loss of a radio wave and increase a delivery distance of a radio wave at the super-high frequency band, various techniques such as a beamforming, a massive MIMO, a full dimensional MIMO (FD-MIMO), an array antenna, an analog beam-forming, and a large scale antenna are discussed in the 5G communication system.

Additionally, for an improvement in network of the 5G communication system, technical developments are made in an advanced small cell, a cloud radio access network (cloud RAN), an ultra-dense network, a device to device (D2D) communication, a wireless backhaul, a moving network, a cooperative communication, coordinated multi-points (CoMP), a reception interference cancellation, and the like.

Besides, in the 5G communication system, a hybrid FSK and QAM modulation (FQAM) and a sliding window superposition coding (SWSC) are developed as advanced coding modulation (ACM) schemes, and a filter bank multi carrier (FBMC), a non orthogonal multiple access (NOMA), and a sparse code multiple access (SCMA) are also developed as advanced access techniques.

System information refers to essential information for communication between a terminal and a base station. In 3GPP LTE, the system information is divided into an MIB (Master Information Block) and an SIB (System Information Block). The MIB is the most essential information. The SIB is subdivided into SIB-x forms according to its importance or cycle. The MIB is transmitted through a PBCH (Physical Broadcast Channel) which is a physical channel. The SIB is common control information and is transmitted through a PDCCH differently from the MIB.

SUMMARY OF THE INVENTION

Meanwhile, the number of system information blocks is continuously increasing, and radio resources are required to broadcast a system information block. Thus, as the number of system information blocks increases, the quantity of radio resources required to broadcast a system information block also inevitably increases. To transmit continuously increasing system information to a user equipment (UE), it is necessary to propose a method for acquiring system information that efficiently utilizes radio resources.

According to an embodiment, there is provided a method for requesting, by a UE, a system information block (SIB) based on a timer in a wireless communication system. The method may include: determining whether an essential SIB is missing from a cell; requesting the missing essential SIB from an RAN when it is determined that the essential SIB is missing from the cell; starting a system information request timer; and re-requesting the missing essential SIB from the RAN when the system information request timer expires.

The essential SIB may be an SIB that the UE needs to obtain in order to perform data transmission and reception in the cell. The missing essential SIB may be an SIB that is not broadcast in the cell.

The essential SIB may include at least one of SIB1 or SIB2.

The method may further include considering the cell as a barred cell when the UE requests the missing essential SIB from the RAN a predetermined number of times.

The method may further include starting a system information prohibit timer upon receiving the missing essential SIB from the RAN before the system information request timer expires.

The method may further include restarting the system information request timer when the system information request timer expires. The method may further include starting a system information prohibit timer upon receiving the missing essential SIB from the RAN before the restarted system information request timer expires.

The system information request timer may be started after the missing essential SIB is requested by the UE. Alternatively, the system information request timer may be started before the missing essential SIB is requested by the UE.

An operation period of the system information request timer may be set by a network per UE, per cell, per area including a plurality of cells, or per public land mobile network (PLMN).

The system information request timer may be set only for a particular type of SIB, a particular type of UE, a UE supporting a particular service, a UE supporting a particular application, a UE configured with a particular service, or a UE configured with a particular application. The particular type of UE may be one of an MTC UE or an NB-IoT UE.

According to another embodiment, there is provided a UE for requesting an SIB based on a timer in a wireless communication system. The UE may include: a memory; a transceiver; and a processor to connect the memory and the transceiver, wherein the processor may be configured to: determine whether an essential SIB is missing from a cell; control the transceiver to request the missing essential SIB from an RAN when it is determined that the essential SIB is missing from the cell; start a system information request timer; and control the transceiver to re-request the missing essential SIB from the RAN when the system information request timer expires.

A UE may selectively receive a missing essential SIB.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2(a) shows a control plane of a radio interface protocol of an LTE system.

FIG. 2(b) shows a user plane of a radio interface protocol of an LTE system.

FIG. 3 shows an example of transmitting a master information block (MIB), system information block1 (SIB1 ), and other SIBs.

FIG. 4 shows an update of system information.

FIG. 5 shows an access class barring (ACB) operation.

FIG. 6A and FIG. 6B show an ACB operation in case of an emergency call.

FIG. 7 shows a non-centralized deployment scenario.

FIG. 8 shows a ‘co-sited deployment with E-UTRA’ scenario.

FIG. 9 shows a centralized deployment scenario.

FIG. 10 illustrates a procedure in which a UE requests an essential SIB based on a timer according to an embodiment of the present invention.

FIG. 11 illustrates a procedure in which a UE requests an essential SIB based on a timer according to an embodiment of the present invention.

FIG. 12 illustrates a procedure in which a UE requests an essential SIB based on a timer according to an embodiment of the present invention.

FIG. 13 illustrates a procedure in which a UE requests system information based on a timer configuration according to an embodiment of the present invention.

FIG. 14 illustrates a procedure in which a UE requests system information based on a location according to an embodiment of the present invention.

FIG. 15 is a block diagram illustrating a method in which a UE requests an SIB based on a timer according to an embodiment of the present invention.

FIG. 16 is a block diagram illustrating a wireless communication system according to the embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below can be used in various wireless communication systems such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), etc. The CDMA can be implemented with a radio technology such as universal terrestrial radio access (UTRA) or CDMA-2000. The TDMA can be implemented with a radio technology such as global system for mobile communications (GSM)/general packet ratio service (GPRS)/enhanced data rate for GSM evolution (EDGE). The OFDMA can be implemented with a radio technology such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc. IEEE 802.16m is evolved from IEEE 802.16e, and provides backward compatibility with a system based on the IEEE 802.16e. The UTRA is a part of a universal mobile telecommunication system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA in a downlink and uses the SC-FDMA in an uplink. LTE-advanced (LTE-A) is an evolution of the LTE. 5G is an evolution of the LTE-A.

For clarity, the following description will focus on LTE-A. However, technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network is widely deployed to provide a variety of communication services such as voice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or more user equipment (UE; 10), an evolved-UMTS terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers to a communication equipment carried by a user. The UE 10 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and a plurality of UEs may be located in one cell. The eNB 20 provides an end point of a control plane and a user plane to the UE 10. The eNB 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as a base station (BS), a base transceiver system (BTS), an access point, etc. One eNB 20 may be deployed per cell. There are one or more cells within the coverage of the eNB 20. A single cell is configured to have one of bandwidths selected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlink or uplink transmission services to several UEs. In this case, different cells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 to the UE 10, and an uplink (UL) denotes communication from the UE 10 to the eNB 20. In the DL, a transmitter may be a part of the eNB 20, and a receiver may be a part of the UE 10. In the UL, the transmitter may be a part of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in charge of control plane functions, and a system architecture evolution (SAE) gateway (S-GW) which is in charge of user plane functions. The MME/S-GW 30 may be positioned at the end of the network and connected to an external network. The MME has UE access information or UE capability information, and such information may be primarily used in UE mobility management. The S-GW is a gateway of which an endpoint is an E-UTRAN. The MME/S-GW 30 provides an end point of a session and mobility management function for the UE 10. The EPC may further include a packet data network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which an endpoint is a PDN.

The MME provides various functions including non-access stratum (NAS) signaling to eNBs 20, NAS signaling security, access stratum (AS) security control, Inter core network (CN) node signaling for mobility between 3GPP access networks, idle mode UE reachability (including control and execution of paging retransmission), tracking area list management (for UE in idle and active mode), P-GW and S-GW selection, MME selection for handovers with MME change, serving GPRS support node (SGSN) selection for handovers to 2G or 3G 3GPP access networks, roaming, authentication, bearer management functions including dedicated bearer establishment, support for public warning system (PWS) (which includes earthquake and tsunami warning system (ETWS) and commercial mobile alert system (CMAS)) message transmission. The S-GW host provides assorted functions including per-user based packet filtering (by e.g., deep packet inspection), lawful interception, UE Internet protocol (IP) address allocation, transport level packet marking in the DL, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/S-GW 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used. The UE 10 and the eNB 20 are connected by means of a Uu interface. The eNBs 20 are interconnected by means of an X2 interface. Neighboring eNBs may have a meshed network structure that has the X2 interface. The eNBs 20 are connected to the EPC by means of an S1 interface. The eNBs 20 are connected to the MME by means of an S1-MME interface, and are connected to the S-GW by means of S1-U interface. The S1 interface supports a many-to-many relation between the eNB 20 and the MME/S-GW.

The eNB 20 may perform functions of selection for gateway 30, routing toward the gateway 30 during a radio resource control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of broadcast channel (BCH) information, dynamic allocation of resources to the UEs 10 in both UL and DL, configuration and provisioning of eNB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 may perform functions of paging origination, LTE_IDLE state management, ciphering of the user plane, SAE bearer control, and ciphering and integrity protection of NAS signaling.

FIG. 2(a) shows a control plane of a radio interface protocol of an LTE system. FIG. 2(b) shows a user plane of a radio interface protocol of an LTE system.

Layers of a radio interface protocol between the UE and the E-UTRAN may be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. The radio interface protocol between the UE and the E-UTRAN may be horizontally divided into a physical layer, a data link layer, and a network layer, and may be vertically divided into a control plane (C-plane) which is a protocol stack for control signal transmission and a user plane (U-plane) which is a protocol stack for data information transmission. The layers of the radio interface protocol exist in pairs at the UE and the E-UTRAN, and are in charge of data transmission of the Uu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides a higher layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer, which is a higher layer of the PHY layer, through a transport channel. A physical channel is mapped to the transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. Between different PHY layers, i.e., a PHY layer of a transmitter and a PHY layer of a receiver, data is transferred through the physical channel using radio resources. The physical channel is modulated using an orthogonal frequency division multiplexing (OFDM) scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physical downlink control channel (PDCCH) reports to a UE about resource allocation of a paging channel (PCH) and a downlink shared channel (DL-SCH), and hybrid automatic repeat request (HARQ) information related to the DL-SCH. The PDCCH may carry a UL grant for reporting to the UE about resource allocation of UL transmission. A physical control format indicator channel (PCFICH) reports the number of OFDM symbols used for PDCCHs to the UE, and is transmitted in every subframe. A physical hybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement (ACK)/non-acknowledgement (NACK) signal in response to UL transmission. A physical uplink control channel (PUCCH) carries UL control information such as HARQ ACK/NACK for DL transmission, scheduling request, and CQI. A physical uplink shared channel (PUSCH) carries a UL-uplink shared channel (SCH).

A physical channel consists of a plurality of subframes in time domain and a plurality of subcarriers in frequency domain. One subframe consists of a plurality of symbols in the time domain. One subframe consists of a plurality of resource blocks (RBs). One RB consists of a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use specific subcarriers of specific symbols of a corresponding subframe for a PDCCH. For example, a first symbol of the subframe may be used for the PDCCH. The PDCCH carries dynamic allocated resources, such as a physical resource block (PRB) and modulation and coding scheme (MCS). A transmission time interval (TTI) which is a unit time for data transmission may be equal to a length of one subframe. The length of one subframe may be 1 ms.

The transport channel is classified into a common transport channel and a dedicated transport channel according to whether the channel is shared or not. A DL transport channel for transmitting data from the network to the UE includes a broadcast channel (BCH) for transmitting system information, a paging channel (PCH) for transmitting a paging message, a DL-SCH for transmitting user traffic or control signals, etc. The DL-SCH supports HARQ, dynamic link adaptation by varying the modulation, coding and transmit power, and both dynamic and semi-static resource allocation. The DL-SCH also may enable broadcast in the entire cell and the use of beamforming. The system information carries one or more system information blocks. All system information blocks may be transmitted with the same periodicity. Traffic or control signals of a multimedia broadcast/multicast service (MBMS) may be transmitted through the DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the network includes a random access channel (RACH) for transmitting an initial control message, a UL-SCH for transmitting user traffic or control signals, etc. The UL-SCH supports HARQ and dynamic link adaptation by varying the transmit power and potentially modulation and coding. The UL-SCH also may enable the use of beamforming. The RACH is normally used for initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to a radio link control (RLC) layer, which is a higher layer of the MAC layer, via a logical channel. The MAC layer provides a function of mapping multiple logical channels to multiple transport channels. The MAC layer also provides a function of logical channel multiplexing by mapping multiple logical channels to a single transport channel A MAC sublayer provides data transfer services on logical channels.

The logical channels are classified into control channels for transferring control plane information and traffic channels for transferring user plane information, according to a type of transmitted information. That is, a set of logical channel types is defined for different data transfer services offered by the MAC layer. The logical channels are located above the transport channel, and are mapped to the transport channels.

The control channels are used for transfer of control plane information only. The control channels provided by the MAC layer include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH) and a dedicated control channel (DCCH). The BCCH is a downlink channel for broadcasting system control information. The PCCH is a downlink channel that transfers paging information and is used when the network does not know the location cell of a UE. The CCCH is used by UEs having no RRC connection with the network. The MCCH is a point-to-multipoint downlink channel used for transmitting MBMS control information from the network to a UE. The DCCH is a point-to-point bi-directional channel used by UEs having an RRC connection that transmits dedicated control information between a UE and the network.

Traffic channels are used for the transfer of user plane information only. The traffic channels provided by the MAC layer include a dedicated traffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCH is a point-to-point channel, dedicated to one UE for the transfer of user information and can exist in both uplink and downlink. The MTCH is a point-to-multipoint downlink channel for transmitting traffic data from the network to the UE.

Uplink connections between logical channels and transport channels include the DCCH that can be mapped to the UL-SCH, the DTCH that can be mapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH. Downlink connections between logical channels and transport channels include the BCCH that can be mapped to the BCH or DL-SCH, the PCCH that can be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, and the DTCH that can be mapped to the DL-SCH, the MCCH that can be mapped to the MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function of adjusting a size of data, so as to be suitable for a lower layer to transmit the data, by concatenating and segmenting the data received from an upper layer in a radio section. In addition, to ensure a variety of quality of service (QoS) required by a radio bearer (RB), the RLC layer provides three operation modes, i.e., a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM). The AM RLC provides a retransmission function through an automatic repeat request (ARQ) for reliable data transmission. Meanwhile, a function of the RLC layer may be implemented with a functional block inside the MAC layer. In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. The PDCP layer provides a function of header compression function that reduces unnecessary control information such that data being transmitted by employing IP packets, such as IPv4 or IPv6, can be efficiently transmitted over a radio interface that has a relatively small bandwidth. The header compression increases transmission efficiency in the radio section by transmitting only necessary information in a header of the data. In addition, the PDCP layer provides a function of security. The function of security includes ciphering which prevents inspection of third parties, and integrity protection which prevents data manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer is located at the lowest portion of the L3, and is only defined in the control plane. The RRC layer takes a role of controlling a radio resource between the UE and the network. For this, the UE and the network exchange an RRC message through the RRC layer. The RRC layer controls logical channels, transport channels, and physical channels in relation to the configuration, reconfiguration, and release of RBs. An RB is a logical path provided by the L1 and L2 for data delivery between the UE and the network. That is, the RB signifies a service provided the L2 for data transmission between the UE and E-UTRAN. The configuration of the RB implies a process for specifying a radio protocol layer and channel properties to provide a particular service and for determining respective detailed parameters and operations. The RB is classified into two types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB is used as a path for transmitting an RRC message in the control plane. The DRB is used as a path for transmitting user data in the user plane.

A Non-Access Stratum (NAS) layer placed over the RRC layer performs functions, such as session management and mobility management.

Referring to FIG. 2(a), the RLC and MAC layers (terminated in the eNB on the network side) may perform functions such as scheduling, automatic repeat request (ARQ), and hybrid automatic repeat request (HARQ). The RRC layer (terminated in the eNB on the network side) may perform functions such as broadcasting, paging, RRC connection management, RB control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway on the network side) may perform functions such as a SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signaling between the gateway and UE.

Referring to FIG. 2(b), the RLC and MAC layers (terminated in the eNB on the network side) may perform the same functions for the control plane. The PDCP layer (terminated in the eNB on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering.

Hereinafter, System Information will be Described.

FIG. 3 shows an example of transmitting a master information block (MIB), system information block1 (SIB1), and other SIBs.

An LTE cell broadcasts basic parameters necessary for the operation of an IDLE_MODE UE and a CONNECTED_MODE UE via a plurality of separate information blocks. Examples of information blocks include an MIB, SIB1, SIB2, and other SIBs (SIBn).

The MIB includes the most essential parameters needed for a UE to access a cell. Referring to FIG. 4, an MIB message is broadcast through a BCH according to a periodicity of 40 ms, and MIB transmission is repeated in all radio frames within the periodicity of 40 ms. The UE receives an SIB message using the parameters received via the MIB.

There are different types of SIBs.

SIB1 includes pieces of information associated with cell access, and particularly includes scheduling information on other SIB s (SIB2 to SIBn) than SIB1 . SIBs having the same transmission periodicity among the SIBs other than SIB1 are transferred via the same system information (SI) message. Thus, scheduling information includes a mapping relationship between each SIB and an SI message. An SI message is transmitted within an SI window in a time domain, and each SI message is associated with one SI window. Since SI windows for different pieces of SI do not overlap, only one SI message is transmitted within an SI window. Thus, scheduling information includes the duration of an SI window and an SI transmission periodicity. Time/frequency for transmitting an SI message is determined by dynamic scheduling by a BS. SIB1 is broadcast through a downlink shared channel (DL SCH) according to a periodicity of eight radio frames (that is, 80-ms periodicity), and SIB1 is repeatedly retransmitted on a fifth subframe of an SFN-mod-2 radio frame within the 80-ms periodicity.

SIB2 includes necessary information for a UE to access a cell. SIB2 includes information on an uplink cell bandwidth, a random access parameter, and an uplink power control parameter.

SIB3 includes cell reselection information. SIB4 includes frequency information on a serving cell and intra-frequency information on a neighboring cell for cell reselection. SIBS includes frequency information on a different E-UTRA and inter-frequency information on a neighboring cell for cell reselection. SIB6 includes frequency information on a UTRA and information on a UTRA neighboring cell for cell reselection. SIB7 includes frequency information on a GERAN for cell reselection. SIB8 includes information on a neighboring cell.

SIB9 includes a Home eNodeB (HeNB) identifier (ID). SIB10 to SIB12 include a public warning message, for example, for earthquake warning. SIB14 is used to support enhanced access barring and controls UEs to access a cell. SIB15 includes information needed to receive an MBMS at contiguous carrier frequencies. SIB16 include GPS time and coordinated universal time (UTC)-related information. SIB17 includes RAN auxiliary information.

Not all SIBs are always required to be present. For example, SIB9 is not needed in a mode where a wireless carrier establishes an HeNB, while SIB13 is not needed if a cell provides no MBMS.

System information is commonly applied to all UEs accessing a cell, and UEs need to always maintain up-to-date system information to perform an appropriate operation. When system information is changed, UEs need to know in advance the time the BS transmits new system information. In order that a BS and a UE mutually recognize a radio frame period for transmitting new system information, the concept of BCCH modification period is introduced in “3GPP TS 36.331 v9. 3.0,” which is described in detail.

FIG. 4 shows an update of system information.

Referring to FIG. 4, a BS, which intends to update system information in an (n+1)th modification period, notifies in advance UEs of an update of system information in an nth modification period. A UE, which is notified the update of the system information in the nth modification period, receives and applies new system information at the very beginning of the (n+1)th modification period. When an update of system information is scheduled, the BS includes a system information modification indicator in a paging message. Generally, a paging message is a message received by an idle-mode UE. However, since an update of system information is notified through a paging message, a connected-mode UE also needs to receive a paging message at times and to identify an update of system information.

Hereinafter, Access Class Barring (ACB) will be Described.

FIG. 5 shows an ACB operation.

Referring to FIG. 5, ACB is a technique for controlling access of a UE to control system load, which includes a stage in which an eNB broadcasts barring information to UEs in advance and a stage in which UEs attempt to perform access based on a class that the UEs have and the barring information broadcast by the eNB 20. When new ACB information is triggered (S501), the eNB 20 may notify an UE AS 11 that SIB (SystemInformationBlock) information is to be changed soon due to the new ACB information, using a systemInfoModification IE of a paging (S502). The UE receiving the paging may recognize that the SIB information to be changed soon (S503). The systemInfoModication IE included in the paging is an indicator indicating that the SIB information is changed, and the indicator may have different forms according to embodiments.

The eNB 20 may notify the UE that the SIB information is to be changed soon through a paging in a modification period before broadcasting changed SIB information. Thereafter, the changed SIB information is broadcast in a next modification period.

After completing the modification period in which the paging is received, the UE receives SIB1 information to receive new SIB information (S504). There are several types of SIBs depending on purposes. SIB1 to SIB14 exist in Rel-11 LTE technology, and SIBs are continuously developed to support new functions of standard technologies. Among different SIBs, SIB1 may include scheduling information on other SIBs. Therefore, SIB1 needs to be received first in order to receive other SIB information.

Subsequently, the UE may receive SIB2 information including ACB information (S505). The UE AS 11 may store the ACB information (S506). A UE NAS 12 may send a service request to the UE AS 11 when a service, that is, communication, is needed (S507). Then, the UE AS 11 may determine whether to allow access based on the stored ACB information (S508). A UE is necessarily assigned a class with a value between 0 and 9. Further, the UE may further be assigned a class having a value of 11 to 15 for a special purpose. In addition, there is class 10 associated with an emergency call. The eNB 20 may restrict access to a particular class. In LTE technology, access to any designated one of classes 0 to 9 may not be restricted, and access to one or more designated classes among classes 11 to 15 for special purposes may be restricted.

Instead, the eNB 20 may provide a barring factor and barring time information with respect to classes 0 to 9. The barring factor may be a value ranging from 0 to 1. The UE selects one random value between 0 and 1 and compares the value with the barring factor. When the value selected by the UE is lower than the barring factor, it may be determined that access is allowed. When the value selected by the UE is higher than the barring factor, access is not allowed and the UE may notify the UE NAS 12 that access is not allowed (S509). When access is not allowed, the UE AS 11 may calculate a Tbarring value using the barring time information and the following equation. The following Equation 1 shows a method of calculating the Tbarring value according to the embodiment. A different Tbarring value may be calculated depending on an embodiment. Tbarring=(0.7+0.6×rand)×ac−BarringTime  [Equation 1]

After Tbarring time, the UE AS 11 may notify the UE NAS 12 that it is possible to reattempt access (S510). Here, the UE NAS 12 reattempts to access the UE AS 11 (S511), and the UE AS 11 may perform a barring check again (S512). The process described in FIG. 5 may correspond to a general mobile originating (MO) call or signaling, that is, a service request triggered by a UE.

An emergency call triggered by a UE is slightly different from the above process. ACB information for an emergency call differs from an MO call or signaling. That is, ac-barringForEmergency information is provided instead of a barring factor or barring time information. The IE may indicate only whether an emergency call is allowed in a Boolean type. When the IE is set to true, the IE indicates that no emergency call is allowed, and access may finally be denied after checking a predetermined additional condition. Also, since no barring time information is provided, there is no stage in which the UE AS 11 notifies the UE NAS 12 whether access is possible after Tbarring. An ACB operation for an emergency call is described in more detail in FIG. 6.

FIG. 6A and FIG. 6B show an ACB operation in case of an emergency call.

Referring to FIGS. 6A and 6B, a UE AS may receive SIB2 information broadcast by an eNB (S600). SIB2 may include an ac-BarringInfo IE including barring information. The UE AS may determine whether there is a request for an emergency call from a UE NAS (S610). When an emergency call is triggered, the UE AS may determine whether an ac-BarringForEmergency IE included in the ac-BarringInfo IE is set to true (S620). When the ac-BarringForEmergency IE is not true, the UE AS may regard the call as being allowed (S640). However, when the ac-BarringForEmergency IE is true, the UE AS may determine whether the UE further has one or more of classes 11 to 15 for special purposes, in addition to classes of 0 to 9 (S630).

When the UE does not have one or more of classes 11 to 15 for special purposes, the UE AS may consider that the call is not allowed (S670), and may notify the UE NAS that the call is not allowed (S680). When the UE has any one class for a special purpose, the UE AS may determine barring information on classes 11 to 15 included in the ac-BarringInfo IE (S650). In an embodiment, information in 5-bit bitmap format indicating whether to bar each special-purpose class may be included in the ac-BarringInfo IE. An ac-BarringForMO-Data IE in the ac-BarringInfo IE may have a 5-bit ac-BarringForSpecicalAC IE. In an embodiment, the respective bits may be sequentially mapped to AC11 to 15. When each bit is set to ‘1’, a corresponding AC is considered to be barred. In this case, AC 12, 13, and 14 are valid in a home country, and AC 11 and 15 are valid only in an HPLMN/HEPLMN. When any one of the special-purpose classes possessed by the UE is allowed to access, access to the call may be regarded as being allowed (S660). Subsequently, the UE may attempt a random access process to attempt a connection with the eNB.

Hereinafter, a 5G RAN Deployment Scenario will be Described.

A 5G RAN may be classified into a ‘non-centralized deployment’ scenario, a ‘co-sited deployment with E-UTRA’ scenario, and a ‘centralized deployment’ scenario according to a shape of deploying a function of a base station in a central unit and a distributed unit and according to whether it coexists with a 4G base station. In this specification, the 5G RAN, a gNB, a next generation node B, a new RAN, and a new radio base station (NR BS) may imply a newly defined base station for 5G. In addition, a basic function to be supported by the 5G RAN may be defined by Table 1.

TABLE 1 Function group similar to E-UTRAN Function similar Synchronization, Paging, Connection, to E-UTRAN Handover, Load balancing, Radio access network sharing, etc. First new RAN function group Support network Capable of supporting core network slice of RAN Slicing Tight Interworking Dual connectivity, Data flow aggregation function between 4G and 5G eNBs Multi-connectivity Function of simultaneously connecting one New RAN node and multiple New RAN nodes through Data flow combination Support multi-RAT Handover function through new direct handover interface (xX) between eLTE eNB and gNB Second new RAN function group UE Inactive mode Function enabling direct connection when new traffic occurs in UE in a state where a radio access resource is released and a wired connection (gNB-NGC) is established Direct service D2D improvement function Non-3GPP Interworking function between Interworking Non-3GPP(e.g., WLAN) and NR Support Inter-RAT Support handover between E-UTRA handover through and NR through Core Core

FIG. 7 shows a non-centralized deployment scenario.

Referring to FIG. 7, a gNB may be configured in a horizontal manner without being split in a layered manner such as a CU and a DU. In this case, a protocol stack of a full set may be supported in each gNB. The non-centralized deployment scenario may be suitable for a macro cell or indoor hotspot environment. The gNB may be directly connected to another gNB or an eLTE eNB through an inter-BS interface. The gNB may be directly connected to a core network through an RAN-CN interface.

FIG. 8 shows a ‘co-sited deployment with E-UTRA’ scenario.

Referring to FIG. 8, a 5G transmission scheme (e.g., new radio) and a 4G transmission scheme (e.g., E-UTRAN) may be used together in one co-sited deployment. The co-sited deployment scenario may be suitable for an urban macro environment. When a gNB configuration is controlled by utilizing load balancing and multi-connectivity, the co-sited deployment scenario can utilize all frequency resources allocated to 4G/5G, and can extend cell coverage for a subscriber located at a cell boundary by using a low frequency.

FIG. 9 shows a centralized deployment scenario.

Referring to FIG. 9, a gNB may be split into a CU and a DU. That is, the gNB may operate by being split in a layered manner. The CU may perform a function of upper layers of the gNB, and the DU may perform a function of lower layers of the gNB. The centralized deployment scenario may be classified into a high performance transport type and a low performance transport type according to transmission capacity and delay characteristics of a transport device which connects the CU and the DU.

When the transport requires high performance, the CU accommodates many functions from the upper layer to the lower layer, whereas the DU accommodates only relatively a small number of lower layers in comparison with the CU. Therefore, processing of the CU may be overloaded, and it may be difficult to satisfy a requirement for transmission capacity, delay, and synchronization of the transport device. For example, when most of layers (RRC layers to physical layers) are deployed in the CU and only an RF function is deployed to the DU, it is estimated that a transmission band of the transport device is 157 Gbps and a maximum delay is 250 us, and thus the transport device requires an optical network with high capacity and low delay. On the other hand, since a transmission delay is short, when an optimal scheduling scheme is used, there is an advantage in that cooperative communication (e.g., CoMP) between gNBs can be more easily realized.

When the transport requires low performance, the CU accommodates an upper layer protocol function having a slightly low processing load, and thus there is room in the transmission capacity and delay of the transport device. For example, when only at least the upper layer (RRC layer) is deployed in the CU and all lower layers (PDCP layer to RF) lower than that are deployed in the DU, it is estimated that the transmission band of the transport device is 3 to 4 Gbps and the maximum delay is 10 ms. Therefore, there is room in the transmission band and the delay in comparison with the transport requiring high performance.

Meanwhile, the number of system information blocks is continuously increasing, and radio resources are required to broadcast a system information block. Thus, as the number of system information blocks increases, the quantity of radio resources required to broadcast a system information block also inevitably increases. To solve this problem, a new type of system information proposed. The new type of system information is not always broadcast by a network but may be transmitted from the network only when a UE request the system information. This type of system information may be referred to as an on-demand system information (OSI) or minimum system information (MSI).

Regarding on-demand system information, a UE may request system information in a cell, and a network receiving the request may transmit the requested system information to the UE. However, when the UE frequently requests system information, an uplink may become very congested. Therefore, it is necessary to newly propose a system information request procedure for minimizing uplink congestion when a UE requests system information. Hereinafter, a method for a UE to request a system information block and a device supporting the same will be described in detail according to an embodiment of the present invention.

In this specification, an RAN may not broadcast all system information in a cell. Alternatively, the RAN may broadcast some of the system information in the cell. The RAN may broadcast only one or more master information blocks (MIBs) and system information blocks of type 1 to type N−1. However, the RAN may not broadcast system information blocks of type N to type M. N and M are system information type numbers, where N is a system information type number lower than or equal to M, and M is the highest system information type number among the system information blocks supported by the cell.

In this specification, a system information request may be transmitted via at least one of the following control signaling:

-   -   a physical-layer resource, such as a PUCCH resource or a PRACH         preamble resource, for example, a random access preamble ID         (RAPID) or a time/frequency PRACH resource;     -   a layer-2 control signaling, such as an MAC control element, an         RLC control PDU, or a PDCP control PDU;     -   an RRC message; and     -   an NAS message.

In this specification, the system information request may include at least one of the following elements:

-   -   a value tag indicating system information currently stored in a         UE;     -   the type number of a system information block requested;     -   a set of system information blocks requested;     -   UE identity;     -   part or all of UE capability information; and     -   support of a particular feature, for example, support of eMBMS         or support of sidelink.

In this specification, the RAN may be an LTE base station (i.e., eNB). Alternatively, the RAN may be a new RAT base station that may include a central unit and/or one or more distributed units. The RAN can manage a cell.

FIG. 10 illustrates a procedure in which a UE requests an essential system information block (SIB) based on a timer according to an embodiment of the present invention.

A cell may not broadcast an essential SIB. The essential SIB may refer to an SIB that a UE needs to obtain in the cell. For example, the essential SIB may be SIB1 . Alternatively, the essential SIB may be SIB1 and SIB2 . Alternatively, the essential SIB may be SIB1 to SIBn. For example, the essential SIB may refer to an SIB that the UE needs to obtain in order to perform data transmission and reception in the cell.

However, when the cell broadcasts all essential SIBs but does not broadcast an SIB associated with a certain feature, the UE does not transmit a system information request. For example, when the cell broadcasts all essential SIBs but does not broadcast an SIB associated with a certain feature, the UE does not transmit a system information request while the UE is in an RRC-idle state. For example, when the cell broadcasts all essential SIBs but does not broadcast an SIB associated with a certain feature, the UE does not transmit a system information request while the UE does not use the certain feature. The certain feature may be an MBMS, D2D, V2X, or a WLAN.

Referring to FIG. 10, in step S1010, the UE may receive system information from an RAN. The system information may be system information broadcast by the RAN.

In step S1020, the UE may determine whether an essential SIB is missing. For example, the UE may determine whether one or more essential SIBs are not broadcast in a cell.

In step S1030, when it is determined that one or more essential SIBs are not broadcast in the cell, the UE may transmit a first system information request to the RAN. The first system information request may selectively indicate to the RAN that the essential SIBs are missing. The first system information request may be a system information request that the UE transmits first after detecting that the essential SIBs are missing. The first system information request may be transmitted after a timer is started. Alternatively, the first system information request may be transmitted before the timer is started.

Before the started timer expires, the UE may receive the missing essential SIBs from the RAN despite the first system information request. On the other hand, the RAN may not transmit the missing essential SIBs to the UE until the started timer expires. In this case, the UE may request the system information again.

In step S1040, the started timer may expire. The UE may not receive the missing essential SIBs until the timer expires.

In step S1050, when the UE does not receive the missing essential SIBs from the RAN despite the first system information request, the UE may transmit a second system information request to the RAN. The second system information request may selectively indicate to the RAN that the essential SIBs are missing. The second system information request may be a system information request that the UE transmits second after detecting that the essential SIBs are missing. The second system information request may be transmitted after a timer is restarted. Alternatively, the second system information request may be transmitted before the timer is restarted.

FIG. 11 illustrates a procedure in which a UE requests an essential SIB based on a timer according to an embodiment of the present invention.

In the embodiment of FIG. 11, it is assumed that the UE detects that an essential SIB is missing.

Referring to FIG. 11, in step S1110, a timer may be started.

In step S1120, the UE may transmit a first system information request to an RAN. The first system information request may selectively indicate to the RAN that the essential SIB is missing. The first system information request may be a system information request that the UE transmits first after detecting that the essential SIB is missing. The order of steps S1110 and S1120 may be changed.

In step S1130, the started timer may expire. The UE may not receive the missing essential SIB from the RAN despite the first system information request until the timer expires.

In step S1140, the timer may be started.

In step S1150, the UE may transmit an Nth system information request to an RAN. The Nth system information request may selectively indicate to the RAN that the essential SIB is missing. The Nth system information request may be a system information request that the UE transmits for the nth time after detecting that the essential SIB is missing. The order of steps S1140 and S1150 may be changed.

In step S1160, the started timer may expire. The UE may not receive the missing essential SIB from the RAN despite the nth system information request until the timer expires.

When the UE is unable to receive the missing essential SIB from the RAN despite the N system information requests, the UE may consider the cell as a barred cell in step S1170. N may be set by the network or may be set by the UE.

According to the embodiment of the present invention, the UE may detect the missing essential SIB and may request the missing essential SIB from the RAN a predetermined number of times. When the UE is unable to receive the missing essential SIB despite requesting the missing essential SIB the predetermined number of times, the UE may consider the cell as a barred cell.

FIG. 12 illustrates a procedure in which a UE requests an essential SIB based on a timer according to an embodiment of the present invention.

In the embodiment of FIG. 12, it is assumed that the UE detects that an essential SIB is missing.

Referring to FIG. 12, in step S1210, a timer may be started.

In step S1220, the UE may transmit a first system information request to an RAN. The first system information request may selectively indicate to the RAN that the essential SIB is missing. The first system information request may be a system information request that the UE transmits first after detecting that the essential SIB is missing. The order of steps S1210 and S1220 may be changed.

In step S1230, the started timer may expire. The UE may not receive the missing essential SIB from the RAN despite the first system information request until the timer expires.

In step S1240, the timer may be started.

In step S1250, the UE may transmit an Nth system information request to an RAN. The Nth system information request may selectively indicate to the RAN that the essential SIB is missing. The Nth system information request may be a system information request that the UE transmits for the nth time after detecting that the essential SIB is missing. The order of steps S1240 and S1250 may be changed.

In step S1260, the UE may receive the missing essential SIB from the RAN. The missing essential SIB may be received from the RAN before the started timer expires.

When the UE receives the missing essential SIB from the RAN, the UE may start a system information prohibit timer in step S1270. While the system information prohibit timer is running, when the UE does not establish an uplink access for another reason, the UE cannot initiate and transmit a system information request. For example, the other reason may be a mobile originating call, mobile originating signaling, or a mobile terminating call.

According to the embodiment of the present invention, the UE may detect the missing essential SIB and may request the missing essential SIB from the RAN a predetermined number of times. When the UE receives the missing essential SIB before requesting the missing essential SIB the predetermined number of times, the UE may operate the system information prohibit timer. Further, while the system information prohibit timer is operating, the UE cannot initiate a system information request.

FIG. 13 illustrates a procedure in which a UE requests system information based on a timer configuration according to an embodiment of the present invention.

Referring to FIG. 13, in step S1310, a network may set either a timer or a period for one or more UEs. The length of the timer or the period may be set per UE, per cell, per area including a plurality of cells, or per public land mobile network (PLMN). The timer may be set only for a particular type of SIB, a particular type of UE, a UE supporting a particular service/application, or a UE configured for a particular service/application. For example, the particular type of UE may be an MTC UE or an NB-IoT UE.

In step S1320, when the timer or the period is set to a value, the UE may periodically initiate and transmit a system information request according to either the timer or the period. For example, the system information request may be initiated and transmitted when the set timer expires.

For example, the timer may be a timer used to periodically update a tracking area. The period may be the length of one BCCH modification period or the length of multiple BCCH modification periods. Since the period may be adjusted to a BCCH modification period, system information may be modified only at the start or end of the BCCH modification period.

FIG. 14 illustrates a procedure in which a UE requests system information based on a location according to an embodiment of the present invention.

Referring to FIG. 14, in step S1410, a network may transmit information on an area to one or more UEs. The information on the area may be transmitted to the UEs through UE-dedicated signaling. Alternatively, the information on the area may be transmitted to the UEs through broadcast signaling, such as system information or paging. The area may be one cell, a set of a plurality of cells, such as a tracking area, or one PLMN.

In step S1420, when camping on or detecting a cell, the UE may detect a change in the area based on the information on the area. The UE may detect the change in the area based on some of system information broadcast from a cell. For example, some of the system information may be a tracking area identity (ID)/code, a PLMN ID, or a cell ID.

In step S1430, when the change in the area is detected, the UE may initiate and transmit a system information request.

When the UE needs to update the location (e.g., cell or tracking area) of the UE (e.g., whenever the UE changes or adds a serving cell or whenever the UE newly changes a tracking area), the UE may initiate and transmit a request message to update the location of the UE or to add a cell. For example, the request message may be a cell update message or a cell addition message. When the UE needs updated system information on a new cell or a new tracking area, the request message may include a system information request. The tracking area may be replaced by any area including one or more cells.

In step S1440, when the request message is received from the UE, the RAN may transmit a response message confirming the update/addition of the location of the UE to the UE. The response message may include system information requested by the UE.

FIG. 15 is a block diagram illustrating a method in which a UE requests an SIB based on a timer according to an embodiment of the present invention.

Referring to FIG. 15, in step S1510, the UE may determine whether an essential SIB is missing from a cell.

The essential SIB may be an SIB that the UE needs to obtain in order to perform data transmission and reception in the cell. The missing essential SIB may be an SIB that is not broadcast in the cell.

The essential SIB may include at least one of SIB1 or SIB2.

When it is determined that the essential SIB is missing from the cell, the UE may request the missing essential SIB from an RAN in step S1520.

Inn step S1530, the UE may start a system information request timer. The system information request timer may be started after the missing essential SIB is requested by the UE. Alternatively, the system information request timer may be started before the missing essential SIB is requested by the UE.

The operation period of the system information request timer may be set by a network per UE, per cell, per area including a plurality of cells, or per PLMN.

The system information request timer may be set only for a particular type of SIB, a particular type of UE, a UE supporting a particular service, a UE supporting a particular application, a UE configured with a particular service, or a UE configured with a particular application. The particular type of UE may be one of an MTC UE or an NB-IoT UE.

In step S1540, when the system information request timer expires, the UE may re-request the missing essential SIB from the RAN.

When the UE requests the missing essential SIB from the RAN a predetermined number of times, the UE may consider the cell as a barred cell. The predefined number of times may be predefined by the network or the UE.

Upon receiving the missing essential SIB from the RAN before the system information request timer expires, the UE may start a system information prohibit timer.

When the system information request timer expires, the UE may restart the system information request timer. Upon receiving the missing essential SIB from the RAN before the restarted system information request timer expires, the UE may start a system information prohibit timer.

FIG. 16 is a block diagram illustrating a wireless communication system according to the embodiment of the present invention.

A BS 1600 includes a processor 1601, a memory 1602 and a transceiver 1603. The memory 1602 is connected to the processor 1601, and stores various types of information for driving the processor 1601. The transceiver 1603 is connected to the processor 1601, and transmits and/or receives radio signals. The processor 1601 implements proposed functions, processes and/or methods. In the above embodiment, an operation of the base station may be implemented by the processor 1601.

A UE 1610 includes a processor 1611, a memory 1612 and a transceiver 1613. The memory 1612 is connected to the processor 1611, and stores various types of information for driving the processor 1611. The transceiver 1613 is connected to the processor 1611, and transmits and/or receives radio signals. The processor 1611 implements proposed functions, processes and/or methods. In the above embodiment, an operation of the UE may be implemented by the processor 1611.

The processor may include an application-specific integrated circuit (ASIC), a separate chipset, a logic circuit, and/or a data processing unit. The memory may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium, and/or other equivalent storage devices. The transceiver may include a base-band circuit for processing a wireless signal. When the embodiment is implemented in software, the aforementioned methods can be implemented with a module (i.e., process, function, etc.) for performing the aforementioned functions. The module may be stored in the memory and may be performed by the processor. The memory may be located inside or outside the processor, and may be coupled to the processor by using various well-known means.

Various methods based on the present specification have been described by referring to drawings and reference numerals given in the drawings based on the aforementioned examples. Although each method describes multiple steps or blocks in a specific order for convenience of explanation, the invention disclosed in the claims is not limited to the order of the steps or blocks, and each step or block can be implemented in a different order, or can be performed simultaneously with other steps or blocks. In addition, those ordinarily skilled in the art can know that the invention is not limited to each of the steps or blocks, and at least one different step can be added or deleted without departing from the scope and spirit of the invention.

The aforementioned embodiment includes various examples. It should be noted that those ordinarily skilled in the art know that all possible combinations of examples cannot be explained, and also know that various combinations can be derived from the technique of the present specification. Therefore, the protection scope of the invention should be determined by combining various examples described in the detailed explanation, without departing from the scope of the following claims. 

What is claimed is:
 1. A method for requesting, by a user equipment (UE), a system information block (SIB) based on a timer in a wireless communication system, the method comprising: determining whether an essential SIB is missing from a cell; requesting the missing essential SIB from an RAN when the UE determines that the essential SIB is missing from the cell; starting a system information request timer; and re-requesting the missing essential SIB from the RAN when the system information request timer expires.
 2. The method of claim 1, wherein the essential SIB is an SIB that the UE needs to obtain for performing data transmission and reception in the cell.
 3. The method of claim 2, wherein the missing essential SIB is an SIB that is not broadcast in the cell.
 4. The method of claim 1, wherein the essential SIB comprises at least one of SIB1 or SIB2.
 5. The method of claim 1, further comprising: considering the cell as a barred cell when the UE requests the missing essential SIB from the RAN a predetermined number of times.
 6. The method of claim 1, further comprising: starting a system information prohibit timer upon receiving the missing essential SIB from the RAN before the system information request timer expires.
 7. The method of claim 1, further comprising: restarting the system information request timer when the system information request timer expires.
 8. The method of claim 7, further comprising: starting a system information prohibit timer upon receiving the missing essential SIB from the RAN before the restarted system information request timer expires.
 9. The method of claim 1, wherein the system information request timer is started after the missing essential SIB is requested by the UE.
 10. The method of claim 1, wherein the system information request timer is started before the missing essential SIB is requested by the UE.
 11. The method of claim 1, wherein an operation period of the system information request timer is set by a network per UE, per cell, per area comprising a plurality of cells, or per public land mobile network (PLMN).
 12. The method of claim 1, wherein the system information request timer is set only for a particular type of SIB, a particular type of UE, a UE supporting a particular service, a UE supporting a particular application, a UE configured with a particular service, or a UE configured with a particular application.
 13. The method of claim 12, wherein the particular type of UE is one of an MTC UE or an NB-IoT UE.
 14. A user equipment (UE) for requesting a system information block (SIB) based on a timer in a wireless communication system, the UE comprising: a memory; a transceiver; and a processor to connect the memory and the transceiver, wherein the processor is configured to: determine whether an essential SIB is missing from a cell; control the transceiver to request the missing essential SIB from an RAN when the UE determines that the essential SIB is missing from the cell; start a system information request timer; and control the transceiver to re-request the missing essential SIB from the RAN when the system information request timer expires.
 15. The UE of claim 14, wherein the essential SIB is an SIB that the UE needs to obtain for performing data transmission and reception in the cell.
 16. The UE of claim 15, wherein the missing essential SIB is an SIB that is not broadcast in the cell.
 17. The UE of claim 14, wherein the processor is configured to consider the cell as a barred cell when the UE requests the missing essential SIB from the RAN a predetermined number of times.
 18. The UE of claim 14, wherein the processor is configured to start a system information prohibit timer upon receiving the missing essential SIB from the RAN before the system information request timer expires.
 19. The UE of claim 14, wherein the processor is configured to restart the system information request timer when the system information request timer expires.
 20. The UE of claim 19, wherein the processor is configured to start a system information prohibit timer upon receiving the missing essential SIB from the RAN before the restarted system information request timer expires. 